MEMS Sensors Rev Their Engines

Fast Lane: Delphi ENgineer Bruce Natvig led the team that developed an earpiece sensor that measures dynamic forces impacting a race car driver's head when a crash occurs. The data will help engineers design better restraint systems.

Drivers in the Indy Racing League have a new piece of gear this year, and it isn't under the hood, but in their ears. Embedded in the drivers' radio earpieces is a tiny MEMS sensor system developed by engineers at Delphi (www.delphi.com) that measures the dynamic forces applied to the driver's head during an accident. The g-force data collected will provide researchers a clearer picture of what happens in that split-second of time that it takes for a crash to occur, leading to better design of driver restraint systems and safety devices.

Size mattered, since the sensing system—consisting of 3 accelerometers, one for each axis, and two circuit boards—had to fit inside the earpiece alongside the transducer for the radio receiver. "Engineers first tried putting the sensors into the helmet, but discovered that the helmet moves relative to the head. It did not produce an accurate reading of head forces," says Bruce Natvig, an engineer at Delphi who headed up the development project.

Small Stuff: The small size of MEMS accelerometers enables them to fit inany application.

Fortunately, the tiny 4.5 x 4.5 x 2-mm sensor system fit comfortably inside an earpiece, sized even for the smallest ear because of the technology of microelectromechanical systems (MEMS), which has made sensors very small. That size opens up a host of new applications.

MEMS devices are essentially mechanical systems in extreme miniature. In their packages, they look like ordinary integrated circuits, but inside they're mechanical systems with extremely small masses, beams, and springs that move. The proof mass in Analog Devices' MEMS gyroscopes, for example, weighs only eight millionths of a gram. It's suspended only two microns (two millionths of a meter) over the device's electronic circuitry. The proof mass in accelerometers from MEMSIC beats even that. It's nothing but a gas that moves in a sealed chamber, and because gas is the only thing that moves, there are no parts that can break. MEMSIC claims its devices can withstand shocks of up to 50,000 g's.

The credit for enabling such small mechanical systems goes to semiconductor manufacturing technology. The same photolithographic processes that create super small transistors are just as capable of creating super small beams, springs, and other mechanical structures. In fact, these mechanical structures, tiny as they are, loom huge in comparison with electronic circuit components. That means that MEMS devices can be produced on trailing-edge semiconductor fabrication equipment, a financial bonus for semiconductor manufacturers. In addition, semiconductor manufacturing processes create hundreds or thousands of MEMS chips at a time on a single silicon wafer, bringing about enormous per-unit cost savings that will only increase as semiconductor manufacturing technology continues to improve.

Two Paths to Production

Two different approaches—bulk micromachining and surface micromachining—create different kinds of MEMS devices. The bulk micromachining process, normally used to make pressure sensors, creates mechanical structures by etching away material from both sides of a silicon slab that's relatively thick (typically 400 microns). The other approach, surface micromachining, adds the common IC-fabrication steps of depositing and then selectively etching multiple silicon layers that are very thin (typically 2 to 4 microns). Surface-micromachined MEMS devices are typically 20 times smaller, by volume, than bulk-micromachined devices, which are tiny themselves. Surface micromachining is the process used to make accelerometers, but Motorola also turned to surface micromachining to make the pressure sensor in its new tire-pressure monitoring system.

How it works: In a MEMS accelerometer, a moving proof mass shifts capacitive sensing fingers that are attached to it. The resulitng change in the capacitance provides a measure of acceleration. In MEMs accelerometers from MEMSIC below, a heated gas changes position as the device moves. Temperature sensors measure the gas's shift to determine acceleration.

Both bulk micromachining and surface micromachining capitalize on silicon's physical strength to make microscopic mechanical structures. According to Dave Monk, development engineering manager for Motorola's Sensor Products Division, a typical diaphragm thickness in a MEMS pressure sensor is 12 to 25 microns, compared to 75 microns for the thickness of a human hair. "This is silicon, but it can flex," Monk says. "You can flex it with a pencil and see it with the naked eye."

In surface-micromachined MEMS devices, mechanical structures are indeed microscopic. For example, the sensor in the ADXRS150 and ADXRS300 MEMS gyroscopes from Analog Devices measures only 1 x 0.5 mm and yet includes, among other things, 5,000 interleaved capacitive-sensing fingers (beams) that move. In a MEMS accelerometer, says Chris Lemaire, business development engineer at Analog Devices, the device's proof mass moves by mere Angstroms.

How do such small devices work? "Picture a trampoline," says Motorola's Monk. A MEMS accelerometer's proof mass, Monk says, is like the trampoline's surface. Silicon springs connect the proof mass to anchor points. The springs aren't coils, but serpentine cantilever beams, and even though they're made of silicon, they can stretch enough to do their intended job. When the proof mass moves, with control provided by the springs, interleaving capacitive-sensing fingers detect minuscule changes in capacitance, which then get converted electronically to an acceleration reading. In a MEMS gyroscope, or angular rate sensor, a vibrating mass replaces the spinning mass of a traditional gyroscope.

The changes in capacitance that a MEMS device detects are just as tiny as MEMS mechanical components. "We're measuring zeptofarads," says Paul Ganci, product line director for the Inertial MEMS Group at Analog Devices. A zeptofarad is 10-21 farads, Ganci notes—so small that having on-chip circuitry to measure it and process the reading is preferable to off-chip circuitry, which can affect readings. Analog Devices and MEMSIC both have such on-chip circuitry. Other MEMS manufacturers put two chips—one with MEMS mechanical components and one with circuitry—in each MEMS package.

Small Size, Big Market

MEMS applications may be growing, but the technology isn't exactly new. Motorola has sold over 300 million MEMS pressure sensors since 1980. Analog Devices has shipped over 100 million MEMS accelerometers since introducing them in 1991.

And now, MEMS devices are getting set to become even more numerous. Automotive applications, for example, already the largest market for MEMS devices, will use more of them—9.1 per vehicle in 2007, according to market research firm In-Stat/MDR (www.instat.com), up from 5.0 in 2002. MEMS pressure sensors are useful for engine management, braking, fuel measurement, suspension control, tire-pressure monitoring, and more. MEMS accelerometers and angular rate sensors are finding use in active suspensions, rollover detection, and automatic headlight leveling.

Falling prices are even pushing MEMS devices into consumer applications. MEMS accelerometers have recently become available for around $2.50 from Analog Devices and startup MEMSIC (www.memsic.com), an Analog Devices spinoff. STMicroelectronics (www.st.com), a big player in components for consumer electronics, is also getting into MEMS. It plans—along with other MEMS manufacturers—to put them into cell phones and PDAs to give engineers functions they never even knew they needed. Says Benedetto Vigna, manager of MEMS development for ST, "I believe this will be the decade of MEMS inertial sensors for consumer applications."

The Skinny: The silicon diaphragm, left, in a mems snesor is typically 12-25 microns thick.

Part of the impetus of new MEMS applications comes from increasingly sophisticated features. Low-g MEMS accelerometers, for example, now have measurement ranges as small as 1g (the acceleration of gravity), with resolution as fine as 0.001 of full scale. Performing as very sensitive motion and tilt sensors, they're starting to provide one-handed, keyless scrolling of displays on cell phones and PDAs. To scroll a mobile phone's tiny display, you just tilt the phone in the appropriate direction. Three-axis accelerometers are even adding zoom capabilities to the displays. To zoom in, you raise the phone or PDA a bit; to zoom out, you lower it. MEMS gyroscopes—more accurately described as angular rate sensors—are also recently available. One of their uses will be in cars for rollover detection.

MEMS sensors are also prized for their reliability and low-power consumption. The accelerometers from Analog Devices (www.analog.com) that deploy airbags in car crashes exhibit less than one failure per billion hours of operation. Motorola's tire-pressure monitor, including MEMS device and additional circuitry for remote wireless operation, will operate for seven to 10 years on a single, tiny button battery.

And then, there are the really esoteric MEMS devices. Researchers have created microscopic MEMS motors, for example, and minuscule mechanical manipulators that can grasp a single red blood cell. On a more practical level, fluidic MEMS actuators by the millions are serving as printheads in inkjet printers. They accurately dispense ink in amounts as small as four picoliters (four trillionths of a liter).

What's not small about MEMS is the growth potential. As automotive applications for MEMS devices increase, and as consumer applications get ready to take off, other areas also show promise. MEMS optical components, such as those in LCD projectors, are one promising area; another is healthcare, where MEMS blood pressure sensors are already widely used. Some observers even view MEMS as being positioned now where microelectronics was in the 1970s and early 1980s, with nowhere to go but up. If that's true, we may see another instance of something very small becoming very big.

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